519 
OF FLAMES CONTAINIXG SALT VAPOURS. 
so that the potential gradient between them and the lower small electrode was 
anyhow less than 0'4 volt, per centim. This gives for the velocity of the negative 
ions, supposing that they travelled from the lower small electrode to the tops of the 
larger electrodes, which were 0’7 centim. higher up and 1 centim. distant horizontallyj 
206 
0-4 X 0-7 
_ ^ centims. 
740- . 
sec. 
This result agrees quite as well as can be expected with the other two estimates 
made of the velocity of the negative ions due to a potential gradient of 1 volt per 
centim. 
To compare the velocities of the negative ions of various salts some measurements 
were made of the E.M.F. necessary to drag them from an electrode in the flame to 
one placed oj)posite to it, but outside the flame. As I have pJready mentioned, this 
E.M.F. increases rapidly as the distance of the outside electrode from the flame 
is increased. By placing it about 0'5 centim. from the edge of the flame the E.M.F. 
necessary was increased to about 12 'o volts. This necessary E.M.F. was determined 
for the following salts, KCL, NaCL, Li 2 C 03 , KF, KI, KBr, K 2 CO 3 , and was found 
in every case to lie between 12 and 13 volts. Thus it appears that the velocity of 
the negative ions is independent of the nature of the salt from which they are derived 
in the flame. 
( 6 .) The Relative Velocities of the Ions in Hot Air. 
The experimental difficulties in the determinations of the velocities of the ions in 
the flame described above prevented anything more than rough estimations of the 
respective relative velocities being obtained. I, therefore, endeavoured to devise a 
method in v/hich the conditions of the experiments should be simpler and more under 
control. It is clear that the gases of a flame, even when this is kept in as steady a 
state as possible, are not a very suitable medium for accurate work. I, therefore, 
tried treating the electrodes by means of electric currents passing through them, and 
put salt on one or both electrodes. After several trieJs, with various forms of appa¬ 
ratus, this method was abandoned, owing to the difficulty of getting the air i^etween 
the electrodes sufficiently hot to keep the salt vapour from condensing. 
A new apparatus was now constructed, in which a current of very hot air practi¬ 
cally replaced the flame in the experiments already described. The water pump and 
pressure-regulating apjoaratus, already described, were used to supply the air. The 
air was heated by 2 :)assing it through a platinum tube, 50 centims. long and 1'3 cen¬ 
tim. in diameter, maintained at a bright red heat in a Fletcher’s gas-tube furnace. 
The stream of hot air so obtained would heat a jflatinum wire red hot 5 or 6 centims. 
from the end of the tube. To prevent the furnace gases getting into the air at the 
end of the tube, a plane flange of platinum, G centims. in diameter, was fitted to the 
end of the tube. 
